This invention relates to semiconductor devices, and more particularly to a method of programming a electrically-erasable programmable read-only memory (EEPROM) device.
Uncontrolled, or incorrectly controlled programming voltage (Vpp) pulses can reduce the reliability of EEPROMs by overstressing the tunnel oxide. It has been proposed to limit the maximum Vpp, and to vary the rate of increase of Vpp, by using a RC time-constant circuit to provide a waveform that rises rapidly at first, then gradually approaches the maximum; this method is disclosed by Oto el at in IEEE Journal of Solid State Circuits, Oct. 1983, pp. 532-538.
The Oto el al publication discloses a circuit for regulating the high voltage used for programming an EEPROM cell by employing an RC circuit which produces a Vpp voltage having a slope that decreases with time according to an RC time constant. This does not provide the optimum pulse shape, merely an approximation. Separate control of the maximum E field and the program pulse ramp rate are not permitted. Further, the feedback used to force the Vpp pulse to follow the RC curve was made intentionally process-independent; this necessitates use of trimming (as with a laser) at a multiprobe test station to adjust the comparison voltage in manufacture.
It is the principal object of this invention to provide an improved method of programming an EEPROM. Another object is to provide a method of controlling the Vpp voltage used in EEPROM programming in a manner which avoids overstressing the oxide at the tunnel area, yet produces optimum programming speed, and is controlled to take into account changes in supply voltage, threshold voltages, temperature, and the like.